Pumping concrete is often more economical than conventional methods of transporting concrete from a mixing plant to formwork. It enables a continuous feed of concrete to be placed at high speed with access to every part of the site. Concrete pumping is sometimes the only possible placing method where the job is inaccessible or the timescale of the construction work makes slower methods impractical.
The transport and placing of concrete by pump is an increasingly popular method. It is very fast and efficient and results in little waste of concrete. Not all concrete will pump, however, and minor variations in the concrete mix can make an otherwise pumpable mix completely unpumpable. The sand grading is particularly important and variations in grading can rapidly cause unpumpability. Even if only a small part of the mix in the hopper proves to be unpumpable, the pump may become blocked, leading to a time-consuming and expensive delay while the pump is stripped down, and the blockage removed.
A common problem associated with concrete pumps results during the starting phase. The pipes are dry and most likely covered with a thin layer of cement from previous pumpings.
When starting up, the concrete will gradually dehydrate while passing through the dry pipes. This creates a plug before reaching the end of the pipe. It is time consuming to disassemble the pump and remove the plug.
Before pumping concrete, the pump and pipeline must be grouted with about 500 liters of a cement slurry or rich mortar in accordance with the pump manufacturer's recommendations. Pumping should be started immediately after grouting. The aforementioned recommendations typically include the following steps:
1. Pour slurry or mortar into hopper while operating the pump slowly;
2. Discharge excess grout to waste, not into formwork;
3. Retain some grout in the hopper to be remixed with the first hopper full of concrete; and
4. Fit the protecting grill on the hopper before loading concrete.
When pumping downhill, a tight plug of damp cement bags or a sponge rubber ball should precede the grout to ensure that the walls of the pipe are properly lined.
Unfortunately the above procedure results in some new disadvantages. For example, the concrete mixing plant has to mix the cement slurry separately which takes time and money relative to the normal production. Also, present standards and regulation prohibit transport of the cement slurry in the same truck, at the same time, as the construction concrete. In this regard, if regulations are broken, or do not exist, the cement slurry is often put on top of the construction concrete in the lorry. This risks mixing during transport which will negatively influence concrete quality. Additionally, since the cement slurry must not be mixed with the concrete, it must be unpumped on the outside of the mould, and removed as waste.